Development of an Alternative Approach to the Acquisition of Fracture Toughness in Laboratory Specimens Containing Residual Stress

Welding is an essential process in many industries and is used for both the production and repair of nuclear plant, notably pressure vessels and piping. However, traditional welding processes can cause large amounts of residual stress to be generated within the structure. Current methodology for evaluating fracture toughness from specimens containing residual stresses, e.g. BS7448, relies heavily on engineering judgement. This can result in inaccurate, albeit generally conservative, values of fracture toughness being used in defect assessments. The aim of the work presented in this paper is to investigate the use of constraint based fracture mechanics to quantify ‘unique material fracture toughness’ from laboratory specimens containing residual stresses using the ‘apparent fracture toughness’ values derived from standard fracture toughness testing. This is achieved using an analytical knowledge of the effect of residual stress on crack-tip constraint and, if incorporated into fracture toughness methodology, remove the need for unreliable residual stress relaxation methods when using weld coupons for fracture toughness assessments. A novel mechanical method for generating residual stresses in single edge notch bend specimens has been assessed analytically. In this paper, computational analysis of low and high constraint bend specimens, each with and without residual stress, is used to demonstrate the principle and validity of the proposed method.